Microwave dielectric ceramic composition

ABSTRACT

A microwave dielectric ceramic composition exhibiting superior high frequency characteristics, particular temperature coefficients suitable for materials for cellular phones, which composition comprises approximately 0.1 to approximately 0.3 mole fraction of CaTiO 3  having a negative temperature coefficient and approximately 0.9 to approximately 0.7 mole fraction of (Li 1/2  Nd 1/2 )TiO 3  having a positive temperature coefficient, and optionally a predetermined amount of an additive, preferably Nd 2  O 3 , Li 2  CO 3 , Na 2  CO 3  and CeO 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

A microwave dielectric ceramic composition having high dielectric constants, high quality coefficient (Q) values, and small temperature coefficients.

2. Description of the Prior Art

Microwave communication systems such as mobile radio communication, satellite communication and satellite broadcasting systems are currently being rapidly developed. Dielectric ceramics have been extensively utilized in a band-pass filter resonator device or a microwave integrated circuit in these systems, because dielectric resonators contribute to miniaturization, they are light weight, relatively inexpensive, and improve thermal stability.

The most important characteristics required in microwave dielectric materials are:

1. a large dielectric constant (εr) for miniaturization, since the size of resonator is inversely proportional to the square root of dielectric constant;

2. a high Q value (=1/tan δ), to improve the selectivity of resonant frequency (f₀); and

3. a small temperature coefficient of resonant frequency, to obtain temperature stable resonant characteristics.

Known representative microwave dielectric materials are:

1. a composite perovskite type solid solution, such as Ba(Mg_(1/3) Ta_(2/3))O₃, having high Q values (Q×f=200,000) at dielectric constants ranging from 25 to 30. [reference: K. Matsumoto, T. Hiuga, K. Takada and H. Ichimura, "Ba(Mg_(1/2) Ta_(2/3))O₃ Ceramic with Ultra-low Loss at Microwave Frequencies" In Proc. of the Sixth IEEE International Symposium on Applications of Ferroelectrics, pp. 118˜121, (1986)];

2. a simple solid solution, such as (Zn, Sn)TiO₄, having medium Q values (Q×f=50,000) and dielectric constants ranging from 30 to 40 [reference: K. Wakino, K. Minai and H. Tamura "Microwave Characteristics of (Zn,Sn)TiO₄ and PbO--Nd₂ O₃ --TiO₂ Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278-281 (1983)]; and

3. a mixed phase material, such as (Ba,Pb)O--Nd₂ O₃ --TiO₂, having high dielectric constants ranging from about 80 to about 90 but relatively low Q values (Q×f=3,000) [reference: K. Wakino, K. Minai and H. Tamura "Microwave Characteristics of (Zn,Sn)TiO₄ and PbO--Nd₂ O₃ --TiO₂ Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278-281 (1983)].

Recently, (Pb, Ca)ZrO₂ type ceramics with high dielectric constants, having dielectric constants of 100 to 105 and Q values of about 1,000 (Q×f=3,000), have been reported [reference: J. Kato, "Material Produces Small Resonators with High Dielectric Constant", JEE, Sep., 114-118(1991)]

Dielectric ceramics having high dielectric constant are suitable materials for microwave device using electric waves of long wave length and are in great demand for use in devices for cellular phones. However, it is very difficult to develop dielectric materials having temperature coefficients of from negative 30 to positive 30 ppm/°C. which also have high dielectric constants and high Q values. Since materials with high dielectric constants usually have low Q values and large temperature coefficients, it is very difficult to satisfy the three desirable characteristics in a single material system. In fact, a material satisfying the three desirable characteristics simultaneously has not been found.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the aforementioned problems encountered in prior art dielectric materials and to provide a microwave dielectric ceramic composition having a superior in dielectric constant, Q value and temperature coefficient.

In accordance with an aspect of the present invention, the above object is accomplished by providing a microwave dielectric ceramic composition comprising approximately 0.1 to approximately 0.3 mole fraction of CaTiO₃ having a negative temperature coefficient and approximately 0.9 to approximately 0.7 mole fraction of (Li_(1/2) Nd_(1/2))TiO₃ having a positive temperature coefficient.

In accordance with another aspect of the present invention, the above object is achieved by providing a microwave dielectric ceramic composition comprising of 0.1 to 0.3 mole fraction of CaTiO₃ having a negative temperature coefficient and 0.9 to 0.7 mole fraction of (Li_(1/2) Nd_(1/2))TiO₃ having a positive temperature coefficient, and a predetermined amount of an additive selected from the group consisting of Nd₂ O₃, Li₂ CO₃, Na₂ CO₃ and CeO₂.

BRIEF DESCRIPTION OF DRAWINGS

The above and other object, features and advantages of the present invention will be more apparent from the following detailed description taken with reference to the accompanying drawings, in which:

FIG. 1 is a graph showing the change in dielectric constant with the mole fraction of CaTiO₃ in a microwave dielectric ceramic composition consisting of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃ ;

FIG. 2 is a graph showing the change in Q value with mole fraction of CaTiO₃ in a microwave dielectric ceramic composition consisting of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃ ; and

FIG. 3 is a graph showing the change in temperature coefficient with the mole fraction of CaTiO₃ in a microwave dielectric ceramic composition consisting of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the phenomenon that when two dielectric materials having dielectric characteristics different from each other are combined, the dielectric constant and the Q value of the resulting dielectric material frequently exhibits a combination of the characteristics of the respective materials. For example, a material with a positive temperature coefficient (T_(f)) is combined with a material with a negative temperature coefficient (T_(f)), both materials having high dielectric constants and high Q values, in a predetermined mole fraction to prepare a microwave dielectric ceramic composition having superior high frequency characteristics. It is difficult to obtain good high frequency characteristics from materials combined because of differences in sintering temperature and shrinkage. Therefore, it is important to select materials having similar sintering conditions.

In the microwave dielectric ceramic compositions according to the present invention, CaTiO₃ (hereinafter "CT") with a positive T_(f) is combined with (Li_(1/2) Nd_(1/2))TiO₃ (hereinafter "LNT") with a negative T_(f) in a predetermined mole fraction, both having high dielectric constants and high Q values.

CT exhibits a dielectric constant (εr) of 170, Q×f=6,000 (f=3 GHz) and T_(f) =744 ppm/°C. LNT exhibits a constant (εr) of 75, Q×f=2,100 (f=3 GHZ) and T_(f) =-274 ppm/°C. The microwave dielectric magnetic composition according to the present invention has an ideal combined structure, in which there is no solid solubility between CT and LNT.

The mole fractions of CT in the microwave dielectric magnetic composition according to the present invention is preferably on the order of 0.1 to 0.3, and the mole fraction of LNT is on the order of 0.9 to 0.7.

The microwave dielectric ceramic composition according to the present invention may be processed in a conventional manner to produce a sintered body for use in a microwave device.

The dielectric ceramic composition, comprising CT with a T_(f) of 744 ppm/°C. and LNT with T_(f) of -274 ppm/°C., exhibits a superior dielectric constant and high frequency characteristics, and favorable temperature coefficient characteristics, and is particularly useful as a dielectric ceramic material for cellular phones. In addition, the microwave dielectric ceramic composition of the present invention comprising CT and LNT may further comprise a predetermined amount of an additive. Preferred additives include Nd₂ O₃, Na₂ CO₃ and CeO₂.

The preferred embodiments of the present invention will now be further described with reference to specific examples.

EXAMPLE 1

Li₂ CO₃, Nd₂ O₃, TiO₂ and CaTiO₃, all of which have a purity of not less than 99%, were measured to prepare mixtures having the mole fractions given in the following Table 1, added, along with alcohol, to a polyethylene vessel using a zirconia ball mill, and subjected to the treatment of wet mixing and pulverization for 24 hours. The resulting mixed powders were dried at 60° C. for 2 hours in a suction dryer to reduce the demixing effect. The dried powders were calcined at temperatures ranging from 900° to 1,100° C. for 2 to 6 hours. A PVA binding agent was added and the mixture subjected to wet mixing and drying. Thereafter, the resulting dried powders were sieved with a sieve of 325 mesh and charged into dies having a diameter of 15 mm. The powders in the dies were molded under a pressure of 600 kg/cm² into circular plates which were subsequently sintered in a platinum box. Sintering was conducted at temperatures of 1,300°, 1,350° or 1,400° C. for 2.4 hours in the air.

After inserting the resulting sintered samples between two parallel conductive plates of a resonator, the dielectric constants were measured in the resonator in the Hakki and Coleman method and Cavity method employing TE₀₁₁ resonance mode, using a network analyzer (model name: (HP8510B).

Two samples were prepared having the same diameters and different heights, one being three times higher than the other. From the two samples, loadless Q value of the dielectric resonator was measured. The Q values of the samples were obtained with the Kobayashi and Tanaka method in which the surface resistance (R_(s)) of conductive material was gained from the loadless Q value, so as to calculate dielectric loss [reference: Y. Koyabashi and S. Tanaka, "Resonant Modes of a Dielectric Rod Resonator Short-Circuited at Both Ends by Parallel Conducting Plates", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, pp. 1077-1085, (1980)].

The temperature coefficients of resonant frequency were calculated by measuring resonant frequency at 25° and 65° C.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 1.

EXAMPLE 2

The composition prepared in Example 1, comprising CT and LNT in the relation of 0.20:0.80 mole fraction, was added with 0.1 to 1.5% by weight of Nd₂ O₃ to prepare microwave dielectrics.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

EXAMPLE 3

Microwave dielectrics were prepared in a manner similar to that in Example 2, except that 0.1 to 1.0% by weight of Li₂ CO₃ was used instead of 0.1 to 1.5% by weight of Nd₂ O₃.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

EXAMPLE 4

Microwave dielectrics were prepared in a manner similar to that in Example 2, except that 0.1 to 1.0% by weight of Na₂ CO₃ was used instead of 0.1 to 1.5% by weight of Nd₂ O₃.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

EXAMPLE 5

Microwave dielectrics were prepared in a manner similar to that in Example 2, except that 0.1 to 1.0% by weight of CeO₂ was used instead of 0.1 to 1.5% by weight of Nd₂ O₃.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

EXAMPLE 6

The composition prepared in Example 1, comprising CT and LNT in the relation of 0.30:0.70 mole fraction, was added with 0.1 to 1.5% by weight of Nd₂ O₃ to prepare microwave dielectrics.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

EXAMPLE 7

Microwave dielectrics were prepared in a manner similar to that in Example 2, except that 0.1 to 1.0% by weight of CeO₂ was used instead of 0.1 to 1.5% by weight of Nd₂₀ ₃.

The measurement results for dielectric constant, Q value and temperature coefficient in the samples are given as shown in Table 2.

                  TABLE 1                                                          ______________________________________                                         Dielectric characteristics depending on the composition                        change of CT and LNT                                                                        Dielectric Quality  Temperature                                                constant   coeff.   coeff.                                        Composition  (εr)                                                                              (Q)      T.sub.f (ppm/cm.sup.2)                        ______________________________________                                         CaTiO.sub.3 (CT)                                                                            170        2000     +744                                          (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 (LNT)                                                       75         700     -274                                          CT(.15) + LNT(.85)                                                                          104         930      -18                                          CT(.2) + LNT(.8)                                                                            120        1017      +10                                          CT(.25) + LNT(.75)                                                                          123        1070      +53                                          CT(.3) + LNT(.7)                                                                            123        1092      +70                                          CT(.4) + LNT(.6)                                                                            130        1200     +138                                          CT(.5) + LNT(.5)                                                                            135         941     +356                                          CT(.6) + LNT(.4)                                                                            141        1051     +379                                          CT(.7) + LNT(.3)                                                                            146        1350     +410                                          CT(.8) + LNT(.2)                                                                            148        1500     +468                                          ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                         Dielectric characteristics depending on additives to the                       composition of CT and LNT                                                               CT(.20) + LNT(.80)                                                                           CT(.30 + LNT(.70)                                       Additive (wt %)                                                                           εr                                                                             Q      T.sub.f                                                                               εr                                                                           Q    T.sub.f                            ______________________________________                                         None       120     1017   +10    123   1092 +70                                Nd.sub.2 O.sub.3 (0.1)                                                                    119     1040   +10.3  126.5 1120 +78                                Nd.sub.2 O.sub.3 (0.3)                                                                    120     1055   +11.1  127.1 1140 +80                                Nd.sub.2 O.sub.3 (0.5)                                                                    120     1065   +13.0  127.5 1170 +81                                Nd.sub.2 O.sub.3 (1.0)                                                                    120     1017   +13.3  127.5 1230 +81                                Nd.sub.2 O.sub.3 (1.5)                                                                    120     1055   +13.0  127.0 1190 +80                                Li.sub.2 CO.sub.3 (0.1                                                                    119     1040   +8.3                                                 Li.sub.2 CO.sub.3 (0.3)                                                                   118     1045   +7.5                                                 Li.sub.2 CO.sub.3 (0.5)                                                                   117      970   +7.0                                                 Li.sub.2 CO.sub.3 (1.0)                                                                   116      870   +7.3                                                 Na.sub.2 CO.sub.3 (0.1)                                                                   119     1060   +8.0                                                 Nd.sub.2 CO.sub.3 (0.3)                                                                   119     1068   +7.5                                                 Nd.sub.2 CO.sub.3 (0.5)                                                                     118.5 1100   +7.3                                                 Nd.sub.2 CO.sub.3 (1.0)                                                                     118.5 1040   +7.3                                                 CeO.sub.2 (0.1)                                                                           120     1105   -3.1   127.3 1230 +52                                CeO.sub.2 (0.3)                                                                           119     1090   -1.5   126.1 1200 +60                                CeO.sub.2 (0.5)                                                                           118     1085   +3.0   125.1 1170 +71                                CeO.sub.2 (1.0)                                                                           115     1070   +5.3   123.5 1130 +81                                ______________________________________                                    

With reference to the drawings set forth, the characteristics and effects are to be described.

Referring to FIG. 1, the graph shows the relationship between the dielectric constant and CT mole fraction in a microwave dielectric consisting of CT and LNT. As shown, the dielectric constant of the CT-LNT dielectric increases as the mole fraction of CT increases.

Referring to FIG. 2, the graph shows the change of Q value with regard to the mole fraction of CT in a microwave dielectric consisting of CT and LNT. As shown, starting from 700 at which the dielectric consists of only LNT, the Q value increases as the mole fraction of CT increases and decreases when the mole fraction of CT is 0.5. The decrease is caused by the formation of a secondary phase.

Turning now to FIG. 3, the curve shows the relationship between the temperature coefficient and the mole fraction of CT in a microwave dielectric consisting of CT and LNT. As shown, the temperature coefficient moves to the positive side as the mole fraction of CT increase. The negative temperature coefficient of CT and the positive temperature coefficient of LNT are supplemented reciprocally, and when the microwave dielectric comprises CT and LNT, the mole fraction of CT to LNT being 0.2:0.8, there is shown a good temperature coefficient useful for microwave dielectric ceramics for cellular phones.

As described hereinbefore, inventive microwave dielectric ceramics comprising CT (0.2 mole fraction) and LNT (0.8 mole fraction) which is sintered at 1,400° C. for 2 hours, exhibits superior high frequency characteristics, such as εr=120, Q×f=3,051 (f=3 GHz) and T_(f) =+10 ppm/°C.

From the Example, the microwave dielectric ceramic composition containing an additive selected from the group consisting of Nd2CO₃, Li₂ CO₃, Na₂ CO₃ and CeO₂ also exhibits superior high frequency characteristics.

Whilst the present invention has been described with reference to certain preferred embodiments and examples, it will be appreciated by those skilled in the art that numerous variations and modifications are possible without departing from the spirit or scope of the invention as broadly described. 

What is claimed is:
 1. A microwave dielectric ceramic composition, comprising approximately 0.1 to approximately 0.3 mole fraction of CaTiO₃ having a negative temperature coefficient and approximately 0.9 to approximately 0.7 mole fraction of (Li_(1/2) Nd_(1/2))TiO₃ having a positive temperature coefficient.
 2. A microwave dielectric ceramic composition according to claim 1, wherein CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃ have mole fraction of 0.2 and 0.8, respectively.
 3. A microwave dielectric ceramic composition according to claim 1, wherein CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃ have mole fractions of 0.3 and 0.7, respectively.
 4. A microwave dielectric ceramic composition according to claim 2, further comprising approximately 0.1 to approximately 1.5% by weight of Nd₂ O₃ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.
 5. A microwave dielectric ceramic composition according to claim 2, further comprising approximately 0.1 to approximately 1.0% by weight of Li₂ CO₃ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.
 6. A microwave dielectric ceramic composition according to claim 2, further comprising approximately 0.1 to approximately 1.0% by weight of Na₂ CO₃ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.
 7. A microwave dielectric ceramic composition according to claim 2, further comprising approximately 0.1 to approximately 1.0% by weight of CeO₂ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.
 8. A microwave dielectric ceramic composition according to claim 3, further comprising approximately 0.1 to approximately 1.0% by weight of Nd₂ O₃ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃.
 9. A microwave dielectric ceramic composition according to claim 3, further comprising approximately 0.1 to approximately 1.0% by weight of CeO₂ based on the total weight of CaTiO₃ and (Li_(1/2) Nd_(1/2))TiO₃. 